Dogs communicate a lot through their tails, whether it be through furious I’m-so-freakin-happy-to-see-you wagging or the I’m-scared-shitless tail between the legs. But a new study from Italy shows that canines also recognize and respond to wagging in surprising ways, including whether the wagging happens on the left side or right side of a fellow dog.

Earlier, the same research team discovered that dogs wag to the right when they’re happy, like seeing their owners, and to the left when they’re feeling stressed or anxious (like seeing a dog they’re hesitant about). Their prior study showed that left-brain activation produced a wag to the right, while right-brain activation produced a wag to the left — a consequence of left/right asymmetric functionality in the brain. Which wasn’t a complete surprise to the researchers; asymmetries in behavior are widespread in the animal kingdom.

By the observations got the researchers thinking: Are dogs on the receiving end of tail wagging able to decipher and respond to these cues? They performed an experiment to find out.

The mood wags the dog — and vice versa

While closely monitoring their reactions, the researchers showed dogs videos of other dogs with either left- or right- asymmetric tail wagging. They observed that, when dogs saw another dog wagging to the left, their heart rates picked up and they looked anxious. But when the wagging happened on the other side, they stayed perfectly relaxed.

Aside from its bark, humans can understand the intentions and feelings of their dogs through any…
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A signal to others?

In the study, the researchers offer at least two explanations. First, dogs might use tail-wagging direction as a way to figure out the mood or state of a fellow canine, like a ‘‘withdrawal’’ state, and then somehow match that state (emotional transfer). Dogs could also use those signals to warn others — either consciously or unconsciously — of impending danger in the environment.

Alternately, dogs could use this information (i.e. the mental state of their fellow dog), to capitalize on it at the expense of the tail-wagger. For example, they could confidently step in and attempt to dominate an unknown individual who is signaling a withdrawn, passive state.

Of the two theories, the researchers are leaning towards the first interpretation. In both cases, heart rate increases — but behavioral measures showed that dogs were more anxious and stressed when they looked at the stimuli wagging the tail to the left side (right-hemisphere activation).